In mixing silicone rubber compositions, it is common practice in the art to first form a silicone rubber base composition consisting of polydiorganosiloxane gum and inorganic filler as main components. If necessary, various additives are also mixed into the base composition. This base composition can then formed into a silicone rubber molded product by mixing it with an organic peroxide or polyorganohydrogensiloxane and a platinum catalyst, and then heating the mixture.
Typically in the art, the silicone rubber base composition has been produced by mixing the starting material components in a double arm kneader-mixer, an internal mixer, or other batch-type mixer. However, mixers of the these types have low capacity and are unsuitable for mass production or continuous processing. It has been proposed, as described in Japanese Patent Application Laid Open No. 2-102007, first to turn a polydiorganosiloxanegum, an inorganic filler, and auxiliary agents into a powder, and then to continuously mix the components by passing them through a rotating dual-screw unidirectional rotary kneader-extruder. In particular, Japanese Patent Application Laid Open No. 6-313046 describes a process in which a polydiorganosiloxane gum, an inorganic filler, and auxiliary agents are continuously mixed in a dual-screw unidirectional rotary kneader-extruder and then their mixture is again mixed under heating conditions in a counter-rotating dual-screw extruder.
The continuous manufacturing processes of the above references require the use of a dual-screw kneader-extruder with a large length to diameter ratio. Furthermore, inorganic filler materials suitable for the process are limited to the use of such substances as fumed silica, sedimentary silica, or a similar reinforcing silica which does not create problems for the process. The use of an inorganic filler which has higher hardness or grain size, such as a diatomaceous earth powder or a quartz powder, lead to abrasive wear on the surfaces of the screws and cylinders of aforementioned dual-screw mixer-extruders. This results not only in the blackening of the silicone rubber composition during mixing, but also gradually in the decrease of the effect of kneading. Furthermore, in order to decrease a temporal creep hardening of the silicone rubber composition, the conventional process requires that the material inside the dual-screw mixer-extruder be subjected to heat treatment, and this, in turn, leads to an increase in energy consumption.
The present invention provides continuous manufacturing of a silicone rubber base composition by a method which does not require the use of a dual-screw mixer-extruder with a large length to diameter ratio, allows the use of such inorganic fillers as diatomaceous earth power, quartz or a similar large-grain and hard powder without abrasive wear of the screws and cylinders, does not cause blackening of the mixed silicone rubber composition, and does not cause production problems even when filmed silica, sedimentary silica, or a similar reinforcing silican are used in large quantities. Furthermore, the invention provides continuous manufacturing of a mass-like silicon rubber base composition with a process which does not require heat treatment of the material inside the kneader-extruder.